Quest for understanding the fundamentals of Linux Operating System and Embedded Systems: [IPV6] Neighbor Discovery Protocols Introduction

Neighbor Discovery Protocols Introduction

Neighbor Discovery protocol combines Address Resolution Protocol (ARP) and ICMP router discovery and Redirect. With IPv4, we have no means to detect whether or not a neighbor is reachable. With the neighbor discovery protocol, a neighbor unreachability detection mechanism has been defined. Duplicate IP address detection has been implemented, too.

The neighbor discovery protocol consists of five ICMP messages:

1. a pair of Router Solicitation/Router Advertisement messages

2. a pair of Neighbor Solicitation/Neighbor Advertisement messages

3. an ICMP redirect message

Neighbor solicitation and advertisement messages

Neighbor solicitation and advertisement messages enable a node to determine the link-layer address of another node (neighbor) on the same link. (This function is similar to the function provided by the Address Resolution Protocol [ARP] in IPv4.)

For example, node 1 on a link wants to determine the link-layer address of node 2 on the same link. To do so, node 1, the source node, multicasts a neighbor solicitation message.

The neighbor solicitation message, which has a value of 135 in the Type field of the ICMP packet header, contains the following information:

Source address: IPv6 address of node 1 interface that sends the message.
Destination address: solicited-node multicast address (FF02:0:0:0:0:1:FF00::/104) that corresponds the IPv6 address of node 2.
Link-layer address of node 1.
A query for the link-layer address of node 2.

After receiving the neighbor solicitation message from node 1, node 2 replies by sending a neighbor advertisement message, which has a value of 136 in the Type field of the ICMP packet header.

The neighbor solicitation message contains the following information:Source address: IPv6 address of the node 2 interface that sends the message.
Destination address: IPv6 address of node 1.
Link-layer address of node 2.

1. After node 1 receives the neighbor advertisement message from node 2, nodes 1 and 2 can now exchange packets on the link. (equivalent to ARP resolution in IPv4)
2. After the link-layer address of node 2 is determined, node 1 can send neighbor solicitation messages to node 2 to verify that it is reachable. (Neighbor Reachability)

Neighbor advertisement –
1. A response to a Neighbor Solicitation message.
2. A node can also send unsolicited neighbor advertisements to announce a link-layer address change.

The received node would then update the link-layer of the node sending NA into it's neighbor
cache.

This can be viewed in "ip -6 neighbor" command output on a linux machine.

RS — Router Solicitation (ICMPv6 type 133)

When an interface becomes enabled, hosts may send out RSes that request that routers generate Router Advertisements (RAs) immediately rather than at their next scheduled time.

RA — Router Advertisement (ICMPv6 type 134)

Routers advertise their presence together with various link and Internet parameters either periodically, or in response to an RS message. RAs contain prefixes that are used for determining whether another address shares the same link (on-link determination) and/or address configuration, a suggested hop limit value, and so forth.